I am testing a power transformer; input 115volts, linear power supply; I'm guessing that the outputs would only draw a 'hand-full' of amps.

I have 240v feeding through a 100 watt light globe, into a 230v:115v auto tranformer, then into the Primary winding of the unloaded transformer in question.

Two issues make me think that there is a problem in the Primary winding;-
1. Light globe glows rather brightly, far more so than when one of my old valve radios is plugged in. I was expecting a dull glow as there is no current being drawn by any of the secondaries.
2. Multimeter readings of Primary resistance sometimes settles on 9 ohms, but often the reading fluctuates.

Seems to me like there is some shorting out of the Primary winding. What say the experts??????????

Cheers,
Ian

Under normal circumstances, if there's no load on the 115 volt transformer, then it should not be drawing any significant current on its primary side.

Large VA transformers can have significant inrush current, but that's momentary.

The thing with the transformer is that they have inductance rater than resistance.

Even so 9 ohms seems low the only 110V device I have is a Grid dip meter 375 (DC) ohm primary. Even in the 110V Midwests I have not seen a primary that low.

Do remember that as we use 230 -250V the primary of the 110V transformer could draw double the current for the same output as a 240V one

Marc

Thanks GTC and Marc,

You have confirmed my thinking ...... I was hoping that I may have overlooked something .......

These things are not made to be pulled apart ..... for obvious reasons .... but it will be interesting to see what the damage inside looks like ......

Cheers,
Ian

You could also measure the secondary volts of the 115 transformer - if it has shorted primary turns you should expect to see no output

For info:


E.T.C. Sydney
250V to 110V transformer Pri. 500R sec 104R (0.8A)
encased some earth leakage (safe)


Yutaka Electric Mfg. Co Japan
220 to 110V reversible auto transformer 220V side 8R 110V side (3A) 3.7R
Earth leakage etc. passes. No issues.


Make sure that there are no loose wires or dirty contacts when measuring. They still make crap cables & terminals that corrode.

Marc